I'll start with full disclosure. I am a member of the 2nd round of play testers for the Elthos RPG & The Mythos Machine. I have been for years. I am also a player in the author's game world, and I've loved the experience. With the exception of his GMing, and snacks, I haven't been paid for this review.
The Elthos RPG rules book is really a fleshed out background for the web based GM/Player utility that is The Mythos Machine. Although it can stand on its own as a simple fantasy RPG, that is not its primary purpose, or its strength. It's meant to support The Mythos Machine service.
The Mythos Machine is a fully integrated world building tool set. You make the world including its genre, places, races, weapons, armors, skills, powers, monetary units, cultures, campaigns and adventures. It tracks the players' characters, that they create, their stats, their derived stats, and it keeps it all online. It also, if the GM cares to, tracks their experience, and their place on the alignment track. It's deep.
It also has a random encounter generator, based on the creatures you created, and the places that you created. It even tells the GM what they're doing.
It's very deep.
As for the Elthos Rule Book, it's a blend of simplicity, and old world RPG game mechanics. There are just three stats, and one die, but it has traditional things like armor class, levels, and hit points. Personally, when I made a world, I modified the heck out of the rules to suit my taste. Since the core is based on three stats, and the conflict resolution is fairly simple, it wasn't hard to do.
That brings me to my last point. The Mythos Machine isn't fully realized yet because it's supposed to be populated with your worlds. If you want to make stuff for others, for free, and share it, you can do that. If you want to make a world, and sell it through The Mythos Machine, you can do that too.
At the moment, the Elthos Rule Book, and The Mythos Machine, are for people who really enjoy making role-playing worlds. If it takes off, it will also be a way for people who want to shop for a new RPG world. It's definitely worth a look if this is your kind of thing.